Sigh. This is one of those problem PSUs, though not the worst I've ever seen. Click the link and follow along. http://www.newegg.com/Product/ImageGallery.aspx?CurImage=17-171-031-04.jpg&Image=17-171-031-03.jpg%2c17-171-031-04.jpg%2c17-171-031-05.jpg%2c17-171-031-06.jpg%2c17-171-031-07.jpg%2c17-171-031-08.jpg%2c17-171-031-09.jpg%2c17-171-031-10.jpg%2c17-171-031-11.jpg%2c17-171-031-02.jpg&S7ImageFlag=0&WaterMark=1&Item=N82E16817171031&Depa=0&Description=COOLER%20MASTER%20eXtreme%20Power%20Plus%20RS-500-PCAR-A3-US%20500W%20ATX12V%20v2.3%20%20%20%20Power%20Supply
So, take a look at this pic and tell me where it says 500W on the sticker. Give up? It doesn't. If anything, it should say 430W as the 3.3, 5, and both 12v rails are not to exceed 431.5W. It does have only 1 6pin PCIe plug, and (360W / 12V) 30A on the 12V rails. This is a lot of room on the 12V rail for only one PCIe plug. 30A should give you two PCIe plugs. This PSU also has a red voltage select switch meaning its built on an old design. This means its <80% efficient at converting AC to DC electricity.
That said you still have 360W. Assuming 100W for your CPU, 50W for your other stuff, that still leaves you 210W for your GPU. The 6870 I think is around 170W, so your ok to use an adapter. You are VERY close to what your PSU can do however, so upgrading it would be a good idea.